Are mining crytocurrencies sustainable at all?

While I am a fan of cryptocurrencies, I'm also concerned by the huge environmental costs of those cryptocurrencies who rely on mining.
I also heard that the so called "new cryptocurrencies" were not minable.
2018 might be a year where more sustainable cryptocurrencies will eventually become prominent. At least the beginning of a new era. Are there "green cryptocurrencies" and which one.
I would like to share with you the thoughts about it all. This is not a post to troll but to raise the true questions. It is not about whether the cryptocurrencies are good or not.
Some big companies like UBS, Swiss are already adopting the blockchain, so their solution based on the blockchain will eventually be compared to the one they had before, which also has an environmental cost.

P.S: I also mine myself, so this is not a critic against miners.